Motorola M12 Oncore User's Guide download pdf (Page 22)

Chapter 3 – Receiver Description

Motorola GPS Products - Oncore User’s Guide

TIME RAIM ALGORITHM DESCRIPTION (M12 TIMING AND UT

MODELS ONLY)

Time Receiver Autonomous Integrity Monitoring (RAIM) is an algorithm in the Oncore

timing GPS receivers (M12 Timing and UT) that uses redundant satellite

measurements to confirm the integrity of the timing solution. The RAIM approach is

borrowed from the aviation community where integrity monitoring is safety critical.

In most surveying systems and instruments, there are more measurements taken

than are required to compute the solution. The excess measurements are redundant.

A system can use redundant measurements in an averaging scheme to compute a

blended solution that is more robust and accurate than using only the minimum

number of measurements required. Once a solution is computed, the measurements

can be inspected for blunders. This is the essence of Time RAIM. In order to perform

precise timing, the GPS receiver position is determined and then the receiver is put

into position-hold mode where the receiver no longer solves for position. With the

position known, the time is the only remaining unknown. In order to compute the time,

the GPS receiver only requires one satellite. If multiple satellites are tracked, then the

time solution is based on an average of the satellite measurements.

When the average solution is computed, it is compared to each individual satellite

measurement to screen for blunders. A residual is computed for each satellite by

differencing the solution average and the measurement. If there is a bad

measurement in the set, then the average will be skewed and one of the

measurements will have a large residual.

If the magnitude of the residuals exceeds the expected limit, then an alarm condition

exists and the individual residuals are checked. The magnitude of each residual is

compared with the size of the expected measurement error. If the residual does not

fall within a defined confidence level of the measurement accuracy, then it is flagged

as a blunder. Once a blunder is identified, then it is removed from the solution and the

solution is recomputed and checked again for integrity.

A simple analogy can be used to demonstrate the concept of blunder detection and

removal: a table is measured eight times using a tape measure. The measurements

are recorded in a notebook, but one of the measurements is recorded incorrectly.

The tape measure has 2 mm divisions, so the one sigma reading error is about 1 mm.

This implies that 95% of the measurements should be within 2 mm of truth.

The measurements and residuals are recorded in the table on the following page.

From the residual list, it is clear that trial six was a blunder. With the blunder

removed, the average and residuals are recomputed. This time, the residuals fall

within the expected measurement accuracy.

Revision 5.0 08/30/02

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Motorola M12 Oncore User's Guide Page 22